1. Field of the Invention
This invention relates to a method for working through ground layers for dredging under water ground layers by means of a dredging device, the dredging device comprising a mechanical dredging component with a part operative to contact the ground layers to exert a dredging action to the ground layers in the course of a dredging action, in which method the part is brought into contact with the ground layers and water jets are injected in the area where the mechanical dredging component is operative.
2. Description of the Related Art
In dredging operations with dredgers or excavators of various types, it has become use to inject high pressure water jets into an area in front of the cutting or dredging component. Thereby, the high pressure water jets may be mixed with air or not. The injection of high pressure water jets has particularly been used in combination with suction hopper dredgers when dredging sandy grounds to cause the sandy grounds to fluidize. The main purpose thereof is to enhance the cutting, suction and pumping process in sandy grounds and to cause a stirring-up of the sludge particles in the water in sludge-like grounds, so that the particles can be moved by the ambient natural water currents and the use of transport vehicles can be avoided. The pressures used in this technique lie in the order of magnitude of 10 bar with a tendency to increase the pressure to about 15-20 bar.
From DE-A-3521560, a method is known for digging dry ground layers with a firm hardness such as for example rocks. In the method of DE-A-3521560, the rock like ground layers are digged by means of an excavator equipped with teeth for dredging the ground layers. High pressure water jets impact the grounds to be excavated with a high energy density and impart a cutting action thereto, thus involving the formation of fissures and cracks which can then be split by the sharp side of the teeth of the excavator. Simultaneously, the size of the parts resulting from the digged grounds is reduced, so that the reduced rocks need not be transported and can be left at the digged location. The pressure of the water jets is mostly between 40 and 400 Mpa.
The method disclosed in DE-A-3521560 however concerns the excavation of dry grounds, which cannot be applied to under water dredging just like that. Namely, the impact of high pressure water jets after displacement through water, will be significantly lower than the impact of a high pressure water jet on a dry ground after displacement through the environmental air. In addition to this, the impact of a high pressure water jet on a dry ground being known, its impact on an under water ground layer cannot be predicted just like that, as it will a.o. strongly vary with the pressure of the water jet and the propagation distance through the water.
It is the aim of the present invention to provide a method for dredging under water ground layers in which the mechanical cutting forces applied by the dredging device can be reduced, which allows harder ground types to be dredged with a machine power which would otherwise be used for dredging grounds with a softer constitution, and with which a higher cutting, suction and pressing production can be attained in identical ground types.
The above outlined purposes of the invention can be achieved with the technical features that the dredging action of the dredging component and the injection of the water jets are carried out simultaneously and the water jets are injected at a pressure of at least 20 bar at the position of, through and/or behind the mechanical dredging component.
In the method of this invention, water jets are injected in the area where the mechanical dredging component is operative, the dredging action of the dredging component and the injection of the water jets being carried out simultaneously. Thereby, the water jets are preferably injected at a pressure of at least 20 bar at the position of, through and/or behind the mechanical dredging component.
The simultaneous dredging action of the dredging component and injection of high pressure water jets allows an optimised co-action of both to be obtained. The result of the optimised co-action depends on the type of ground to be dredged and can be summarised as follows. Because of the optimised co-action it becomes possible to enhance in the immediate vicinity of an area of a rock-like material that has been cut by the dredging device hydraulic fracturing in the non-crushed part thereof, to cut open ground layers such as clay layers and/or fluidize ground layers such as sand layers in the vicinity of the cutting or dredging component. The optimised co-action also results herein that broken-off and crushed material can be immediately removed by the high pressure water jets from the location where the mechanical cutting or dredging component is active, in particular in case the ground layers contain rock-like materials or consist virtually or exclusively of rock-like materials such as rock layers.
It has been found that simultaneously with the improved dredging operation of the dredging device, the wear of the dredging components can be reduced, including wear of the teeth thereof. Also, in case of dredging sandy materials, the dredging efficiency can be improved. It has namely been found that when dredging sand grounds, the sand is fluidized by the action of the water jets. The fluidized sand presents the advantage that it can be pumped as a fluid, and not as a water/sand mixture, so that the pump efficiency can be improved.
In the method of this invention, ground layers are understood to include gravel, sand and clay layers or ground layers containing rock-like materials or consisting virtually exclusively of rock masses such as rock layers. Examples of dredging devices suitable for use in the method of this invention include suction hopper dredgers, cutter suction dredgers, bucket dredgers, grab dredgers, pull shovel pontoons or the like. Each of these devices comprises a mechanical cutting or dredging component, part of which comes into contact with the ground and/or rock layers for dredging.
In case the dredging device is a hopper dredger, preferably water jets are also injected to the ground layers to be dredged at a pressure of at least 50 in front of the mechanical dredging component. In that way an optimum fluidization of the soil or an optimum cutting of the clay can be achieved before it enters the draghead.
In specific conditions, in particular when the ground layers contain rock-like materials or consist virtually exclusively of rock-like materials and use is made of a cutter dredger, water jets are injected at pressures of preferably at least 100, preferably from at least 600 to 2000 bar, for example 620 bar. Such a water jet is capable of blowing away the crushed zone that has been created by the mechanical cutting tool.